a) If a transversal intersects two parallel lines, then the alternate angles formed are congruent ===> This is correct.
The alternate exterior and interior angles formed are congruent if a transversal intersects two parallel lines
b) If a transversal intersects two parallel lines, then the corresponding angles formed are congruent. ===> This is also correct.
Corresponding angles formed are congruent if a transversal intersects 2 parallel lines
c) The vertical angles are always congruent. ===> This is also correct.
Therefore all of them are correct.
ANSWER:
d. All of these
